Analysis shows they are lined with a mineral called olivine and contain microscopic diamonds, suggesting they are rare ureilite-type meteorites from an F-class asteroid, a type that makes up 1.3 percent of all known asteroids.

Most asteroids exist today as they did when they first formed 4.6 billion years ago. They still have a lot of their original components—fine grains, specs of cosmic dust, and little molten droplets.

Other asteroid types have gotten "so big and pummeled in collisions that they melted," Jenniskens said. "The iron sinks to the core, and other material floats on top."

The F-class asteroids are in between, melted in parts, but with other areas revealing the materials that made them.

"Our rock still has a lot of the original carbon there, highly cooked," Jenniskens said.

Also, asteroid colors change with distance from the sun. Inner belt asteroids tend to be brighter, and have spectral signatures suggesting different mineral composition than in the outer edge of the belt.

In the simplest scenario, those darker ones are more primitive, noted Lucy McFadden of the University of Maryland, College Park.

However, ureilite sort of contradicts that story, because we know they have been heated, and yet they are dark.

McFadden added that the asteroid appears to be a chunk of a former almost-planet that had to have been hundreds of kilometers in size.

Saving Millions

Observing the asteroid as it approached allowed ample time for astronomers to trace its orbit. They discovered that it shares a similar orbit to another, much larger F-class asteroid, 1998 KU2, which could be the original parent of 2008 TC3.

"These guys have managed for the first time to unequivocally connect the dots between an asteroid and its meteorite samples," said Harry McSween, a geologist and meteorite expert at the University of Tennessee in Knoxville.

The new information, he added, is equivalent to "a free asteroid sample return mission that would [have] cost NASA many millions."